[RE-wrenches] Inverter with two strings of different orientation

[jay peltz]

Hi Peter,

First,  a  possible way for you to confirm some answers in regards to  
the 2 arrays.
set up two arrays in your yard, in whichever orientation you want.   
Then hook up into 1 and then 2 inverters and see the wattage difference.
This would allow you to do lots of metering.

About the MPP in battery based, I don't have any data about the XW- 
MPPT unit.
With the Outback one I have watched my 3 for long periods of time and  
I can say they don't fluctuate outside of normal tracking and sunlight  
variations.

Have you checked out Photon Mag.  They have been doing some great  
testing of inverters.  Expensive mag, but worth it.

all the best,

jay

peltz power

On Dec 29, 2008, at 10:07 AM, Peter Parrish wrote:

> Thanks Bob, these are really great reference articles. The equation  
> for Vmp
> had four terms, three of which are temperature dependent. So I think  
> we put
> that issue to bed. The reason that this issue is so important for us  
> is
> this: if Vmp is temperature dependent, then there must be some loss  
> when
> combining two strings together with different tilts/azimuths. How  
> much, I
> don't know. Specifically, I have a client for whom we are designing  
> a system
> with two strings with different tilts/azimuths (otherwise identical)
> 18deg/East and 15deg/South. The questions is, "Go with one inverter  
> and wire
> the strings in parallel, or go with two inverters one for each  
> string."
> Since the cost for two inverters is considerably more than one  
> inverter of
> twice the capacity, I would like to go with the single inverter, if  
> the
> losses are in the 1-2% range.
>
> I am also very skeptical about MMP tracking algorithms. I documented  
> the
> "saw-toothing" on the Fronius IG-4000 input (July 2006), and we  
> still have
> an outstanding malfunction with a Xantrex XW6048 with an output power
> oscillation.
>
> Right now I will be telling our clients that "using a single  
> inverter --
> with two or more subarrays of widely differing orientations (see  
> above) --
> is not recommended, because there may be significant losses  
> (significantly
> more than 1-2%)".
>
> - Peter
>
> Peter T. Parrish, Ph.D., President
> California Solar Engineering, Inc.
> 820 Cynthia Ave., Los Angeles, CA 90065
> Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
> CA Lic. 854779, NABCEP Cert. 031806-26
> peter.parrish at calsolareng.com
>
> -----Original Message-----
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of boB  
> Gudgel
> Sent: Sunday, December 28, 2008 5:07 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] Inverter with two strings of different
> orientation
>
> Peter Parrish wrote:
>> Bill,
>>
>> I looked into this several years ago, and I was able to get some  
>> digitized
>> I-V data. I can't find that now (but I will). The data supports my  
>> claim.
>>
> Vmp does change with irradiance.  It just changes  very little.  
> Depends
> on your definition of "change" I guess.
> Negligible I would say... That is, assuming a higher irradiance than
> "dark" of course.
>
> I can see where some confusion might come from from irradiance and PV
> parameter variation...  As the irradiance
> changes, the ratio of Vmp to Voc changes.  Lower irradiance normally
> makes the Vmp/Voc rise somewhat but only
> because the Voc is changing.
>
> Now, put two or more modules in series, then shade one of them and
> that's something entirely different.
>
>
> Here's a good paper with some good data on this subject.    I also see
> Bill Brooks contributed to this article,
>
> http://photovoltaics.sandia.gov/docs/PDF/King%20SAND.pdf   (the %20   
> is
> a space)
>
> And for temperatue relations, here's another good one:
>
> http://photovoltaics.sandia.gov/docs/PDF/IEDFB5~1.pdf
>
> boB
>
>
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